Many methods have been reported for preparing oriented nanostructures, but most of these methods cannot be applied to organic polymer materials. Oriented carbon nanotubes are prepared through chemical vapor deposition (CVD). Large arrays of oriented carbon nanotubes were grown from catalyst particles immobilized on porous silica or glass substrates (Li et al., Science, 274, 1996; and Ren et al., Science, 282, 1150, 1998). A gas phase reaction or similar high temperature reactions have been used to prepare oriented nanorods of ZnO (Huanh et al., Science, 292, 1897, 2001), Si (Yu et al., Physica. E., 9, 305, 2001), and silicon carbide/nitride (Chen et al., J. Phys. Chem. of Solids, 62, 1567, 2001). A solution based synthesis method has been developed to prepare oriented nanorods of ZnO2 using a hydrothermal process (Vayssieres et al., Phys. Chem. B, 105, 3350, 2001).
Another widely investigated approach to prepare oriented nanoscale materials is through templated synthesis, in which an inert nonconductive substrate material with oriented nanoporosity is used as the mold or template (Huczko, Appl. Phys. A, 70, 365, 2000). The template nanoporosity is filled with the desired material. Subsequently, the substrate is partially or completely removed leaving a residue of the desired material that replicates the nanostructure of the template. Some of the mostly widely used templates include filtration membranes (e.g., polycarbonate films) and anodic alumina membranes.
The templated method was also used to prepare oriented rods or tubes of polypyrrole and polyaniline (De Vito et al., Chem. Mater., 10, 1738, 1998; and Marinakos et al., Chem. Mater., 10, 1214, 1998). Recently Gao et al. (Angew. Chem. Int. Ed., 39, 3664, 2000), used oriented carbon nanotubes prepared by a CVD process as the template to electrochemically deposit a thin polyaniline polymer coating on the surface of the carbon nanotubes. This method produced a carbon nanotube/polyaniline composite with reportedly good electrical conductivity and electrochemical activity.
Electrospinning has also been used for preparing conducting polymer nanofibers (Doshi et al., J. Electros., 35, 151, 1999; and Reneker et al., J. Appl. Phys., 87, 4531, 2000). In electrospinning, a high voltage is applied to the tip of a syringe until a jet is produced. The charged polymers in the jet repel each other to form thin fibers.
Despite all of these efforts, a need continues to exist for synthesis methods for controlling the morphology of nanostructures, particularly those made from conducting polymers. Ideally, a nanostructure synthesis would be templateless and involve liquid phase processing that can be used as the reaction medium for a wide variety of materials.
One class of conducting material that has attracted increasing attention are polynuclear transition metal hexacyanometallates by virtue of their electronic, electrochemical, and spectrochemical properties. Electrodes formed from films of hexacyanometallates have been made, but their instability and electrical properties remains a critical issue. For example, composite modified electrodes have been made with conducting polymer films that include iron (III) hexacyanoferrate (also known as “Prussian blue”) as a dopant or inorganic conductor (Ogura et al., J. Electrochem. Soc., 142, 4026, 1995; Koncki et al., Anal. Chem., 70, 2544, 1998; and Ikeda et al., J. Electroanal. Chem., 489, 46–54, 2000). Composite films made from polyaniline and iron (III) hexacyanoferrate are described in U.S. Pat. No. 5,282,955 (Leventis et al.). Leventis et al. does not describe a synthesis method for controlling the morphology of composites to produce an oriented nanostructure and the electrochemical deposition of the polyaniline is accomplished by quickly cycling (e.g., from 10–1000 millivolts/second) the electrode between two voltages. Composite films made from poly(3,4-ethylenedioxythiophene) and iron (III) hexacyanoferrate are described in Noel et al., “Composite films of iron (III) hexacyanoferrate and poly(3,4-ethylenedioxythiophene)”, Journal Electroanalytical Chemistry 489, 46–54 (2000). Noel et al. does not describe a synthesis method for controlling the morphology of composites to produce an oriented nanostructure and the electrochemical deposition of the poly(3,4-ethylenedioxythiophene) is accomplished by quickly stepping the voltage to increasingly higher voltages.
One application of iron (III) hexacyanoferrate-modified electrodes is in the construction of biological and chemical sensors. More specifically, there is an increasing need for more sensitive and selective detection or measurement of peroxide compounds in clinical, pharmaceutical, food, industrial, and environmental applications. For example, amperometric determination of hydrogen peroxide is of great importance, inspired by the wide use of peroxide sensors in bioanalytical systems based on oxidase-type enzymes. In oxidase-catalyzed reactions, oxygen and hydrogen peroxide are the substrate and product, respectively. Hydrogen peroxide determination is also important to ensure the safety and quality of pharmaceutical and cosmetic formulations. In addition, monitoring of organic (hydro)peroxides formed during the reaction of ozone with organic compounds in the atmosphere and drinking water or directly released into the environment from numerous industrial processes is desirable because of their adverse health effects.
Amperometric determinations of peroxides are generally performed by oxidation at +0.6 to +0.7 V vs. Ag/AgCl on a platinum electrode (for H2O2) (Guilbault et al., Anal. Chim. Acta 64, 439–455, 1973) or by reduction at −0.3 to −1.0 V vs. Ag/AgCl on gold/mercury amalgam or glassy carbon electrode (for organic and lipid hydroperoxides) (Cosgrove et al., Analyst, 113, 1811–1815, 1988; Funk et al., Anal. Chem., 52, 773–774, 1980). At such large overpotentials, substances present in biological samples such as ascorbic acid, uric acid and acetaminophen interfere under oxidation conditions, while oxygen, benzoquinone, and nitrobenzene interfere at such reduction potentials. Low selectivity, therefore, is a major limitation in amperometric determinations.
One approach for addressing this problem is to use selective electrocatalysts that lower an overpotential of hydrogen peroxide electrooxidation to an appropriate level that prevents the discharge of other substances at the applied electrode potential. Iron (III) hexacyanoferrate has been identified as a possible selective electrocatalyst. For example, Garjonyte et al., Sensors and Actuators B 46, 236–241 (1998) describe a carbon paste electrode modified by ferrous hexacyanoferrate that electrocatalyzed the cathodic reduction of hydrogen peroxide. Karyakin et al., “Prussian Blue-Based First-Generation Biosensor, A Sensitive Amperometric Electrode for Glucose”, Anal. Chem., 67, 2419–2423, 1995, describe a glucose amperometric biosensor made by glucose oxidase immobilization onto a Prussian blue-modified electrode with a perfluorosulfonate ionomer (Nafion® membrane) layer. In the sensors described by Garjonyte et al. and Karyakin et al. the Prussian blue sensing sites are only accessible by the analyte on a two-dimensional electrode surface and, thus, miniaturization of the sensor is difficult due to the limited total sensing surface area.